A semiconductor laser capable of continuous pulsation with a small current is used as a light source in various applications such as in optical communications and a pickup (information reading device) for compact disk players and optical disk memory devices. A package structure of a typical semiconductor laser for accommodating and supporting a semiconductor laser chip therein is shown as denoted by numeral 20 in FIG. 4. The semiconductor laser 20 comprises a disk-like stem 23a formed by processing an alloy material containing cobalt, copper, nickel or the like at a high precision, a bonding post 23b, a semiconductor laser chip 22 fixed on the bonding post 23b in such a manner that a submount 28 is interposed between the semiconductor laser chip 22 and the bonding post 28, and a photodiode 24 for monitoring. The semiconductor laser chip 22 and the photodiode 24 are connected to electrode terminals 25a and 25b, respectively through a gold wire or the like, and embraced and scalded in a cylindrical cap case 27 provided with a light-transmissive window 26 made of, for example, glass and mounted on the stem 23a. The inside of the semiconductor laser 20 is filled with inert gas. As the material for the submount 28, silicon is generally selected because of its excellent heat-releasing property and its thermal expansion coefficient which is approximate to that of a compound semiconductor forming the semiconductor laser chip 22. It is possible to form a photodiode for monitoring in this silicon submount.
In an optical pickup device, using the semiconductor laser 20 as a light source, a laser beam needs to be spotted with accuracy on a pit track formed in the disk face so that a tracking servo mechanism is indispensable. In such a tracking servo mechanism what is referred to as "3-beam method" is widely employed.
In this method, as shown in FIG. 4, the laser beam which is emitted from the semiconductor laser chip 22 is split into three beams through a diffraction grating 31 (in FIG. 4, reflected beams of the upper and lower beams are each split at the diffraction grating, as plotted with dotted line), reflected at a half mirror 32, and converged onto a disk (not shown) through an optical system such as a convex lens 33. The originally emitted beam is referred to as a main beam, while the two other beams produced at the diffraction grating are referred to as subbeams. The difference in light intensity between the subbeams reflected at the disk is detected so as to issue a fine control signal for an optical system comprising lens and the like.
With the 3-beam method, which offers a superior detection accuracy, however, it is known that the main beam reflected at the disk passes through the diffraction grating of the beam splitter while forming subbeams again and returns toward the semiconductor laser chip, or the light source. The returning beam is focused on the beam-emitting face of the semiconductor laser chip or the end face of the submount on the beam-emitting side. When focused on the beam-emitting face, the returning beam is regularly reflected thereat and runs toward the disk again. As a result, the reflected beam of the returning beam interferes with the normal beam so that the tracking servo or the like is made instable.
There have been proposed various countermeasures for preventing such interference due to the returning beam in the case of the 3-beam method. One of such countermeasures is to coat with a nonreflective film or a resin colored black a returning beam incident region of the beam-emitting face of the semiconductor laser chip or that of the end face of the submount on the beam-emitting side. The returning beam incident region is a region onto which the returning beam is focused. However, since the semiconductor laser chip and the submount themselves are very small as described above, this countermeasure has a disadvantage in mass productivity.
As the countermeasures against the returning beam which focuses on the submount, there are known such methods as to cause the returning beam to be reflected in a direction different from the beam-emitting direction by notching the returning beam incident region of the end face of the submount to provide inclination therein (refer to Japanese Unexamined Utility Model Publication No. 151362/1986), and as to cause the returning beam to be reflected in a direction different from the beam-emitting direction by utilizing as the returning beam incident region a cleavage plane of crystal which has an inclination with respect to the principal plane thereof (refer to Japanese Unexamined Patent Publication No. 24488/1989).
In the 3-beam method, as described above, the laser beam emitted from the semiconductor laser chip is split into three beams through the diffraction grating, and the returning beam which is reflected at the disk is also split into three beams when passing through the diffraction grating again. The two subbeams of the returning beam are focused on the upper and lower sides of the beam-emitting face of the semiconductor laser chip, respectively, and the reflected beams of the focused beams interfere with the normal beam thereby making the tracking servo instable. Although the problem caused by the subbeam which is focused on the lower side of the beam-emitting face is overcome by providing an inclination to the returning beam incident region of the submount, the problem caused by the subbeam which is focused on the upper side of the beam-emitting face remains unsolved because the art of notching to provide an inclination in the end face of the semiconductor laser chip or coating the end face with a nonreflective film is unsuitable for mass production. Further, if the semiconductor laser chip is made thinner to avoid such a problem, it is damaged by strain. Hence, the chip cannot be made thinner than 100 .mu.m. For this reason, there are attempts to selectively coat a very small region onto which the subbeam is focused with a nonreflective film or a resin colored black. However, any of such methods is not yet established as a method adaptable for mass production.